guests/linux: Always order discovered network interfaces

plugins/guests/linux/cap/network_interfaces.rb

@@ -12,7 +12,7 @@ def self.network_interfaces(machine, path = "/sbin/ip")
           machine.communicate.sudo("#{path} -o -0 addr | grep -v LOOPBACK | awk '{print $2}' | sed 's/://'") do |type, data|
             s << data if type == :stdout
           end
-          s.split("\n")
+          s.split("\n").sort
         end
       end
     end

test/unit/plugins/guests/linux/cap/network_interfaces_test.rb

@@ -0,0 +1,30 @@
+require_relative "../../../../base"
+
+describe "VagrantPlugins::GuestLinux::Cap::NetworkInterfaces" do
+  let(:caps) do
+    VagrantPlugins::GuestLinux::Plugin
+      .components
+      .guest_capabilities[:linux]
+  end
+
+  let(:machine) { double("machine") }
+  let(:comm) { VagrantTests::DummyCommunicator::Communicator.new(machine) }
+
+  before do
+    allow(machine).to receive(:communicate).and_return(comm)
+  end
+
+  after do
+    comm.verify_expectations!
+  end
+
+  describe ".network_interfaces" do
+    let(:cap){ caps.get(:network_interfaces) }
+
+    it "sorts discovered interfaces" do
+      expect(comm).to receive(:sudo).and_yield(:stdout, "eth1\neth2\neth0")
+      result = cap.network_interfaces(machine)
+      expect(result).to eq(["eth0", "eth1", "eth2"])
+    end
+  end
+end